The production of wool glass fibers by means of the rotary process is well known. In general, molten glass is fed into a spinner which revolves at high speeds. The spinner has a peripheral wall containing a multiplicity of orifices. Molten glass passed by centrifugal force through the orifices of the peripheral wall forms small diameter molten glass streams. Positioned circumferentially about the spinner in an annular blower for turning the fibers downwardly and, in some cases, for further or secondary attenuation of the original or primary fibers to produce fibers of smaller diameter. As the streams of molten glass are emitted from the orifices, they are still sufficiently nonviscous that surface tension forces pull or shape each of the molten streams into substantially circular cross-sections, regardless of the cross-sectional shape of the streams as they are emitted from the orifices. Further, rotary fiberizers are typically equipped with annular burners or other sources of hot gases for secondary attenuation of the primary fibers; these hot gases keep the glass sufficiently fluid or nonviscous that fibers of substantially circular cross-section result.
The production of textile or continuous glass fibers by mechanically drawing molten streams of glass from orifices in the bottom wall of a bushing or feeder is also well known. Non-uniformities in the roundness of the molten streams tend to be corrected by surface tension forces prior to the cooling and hardening of the molten streams into glass fibers. Thus, as in the case of wool glass fiber production, it has not been possible to produce significantly non-circular continuous fibers using shaped orifices in a bushing.
There has long been a need for producing fibers, both in the rotary process and in the continuous fiber process, that have significantly non-circular cross-sections. With respect to reinforcement of resin matrices, such non-circular fibers would be useful in imparting greatly increased transverse strength and improved shear strength qualities. Non-circular fibers for use as insulation materials would be advantageous in that the increased surface area per unit volume of glass would lower the thermal conductivity of insulation made from such fibers.
A measure of the non-circularity of mineral fibers is the "mod ratio", which is defined as the ratio of the diameter of the smallest circle into which the fiber cross-section fits to the diameter of the largest circle which can fit inside the fiber cross-section. As employed herein, fibers having a mod ratio of less than 1.2 are referred to as circular fibers; fibers having a mod ratio greater than or equal to 1.2 are referred to as non-circular fibers.
One attempt to make non-circular glass fibers was by Warthen, as described in U.S. Pat. No. 3,063,094. Warthen's method employs mechanical perturbation of the glass stream while it is still in plastic, deformable state. Warthen teaches that to create a non-circular fiber, the glass stream, initially in a conical shape with a circular cross-section, should be distorted at a region where the viscosity of the stream is sufficiently high as to become rapidly chilled or solidified during attenuation of the streams to a continuous fiber whereby a similar distortion in the cross-section configuration is retained in the attenuated solidified fiber. Warthen also teaches that a heat sink is to be applied to the glass stream by direct contact. This raises the viscosity of the molten glass to better enable retention and perpetuation of the non-circular cross-sectional character of the mechanically perturbed molten glass stream.
Other attempts at producing non-circular fibers have been shown in the art. Processes are known for modifying round glass fibers in order to produce optical fiber wave guides of a non-circular shape. In some cases, a double crucible method is employed for making an optical fiber in which the inner fiber core has a non-circular cross-section. It is also known in the art of making glass fibers to use a mechanical stirrer positioned within the fiber forming cone to produce a twisted or distorted fiber.
In the art of producing organic fibers, it is a common practice to use quenching methods to solidify molten streams of organic material into non-circular cross-sections which are similar to the shapes of the non-circular orifices. However, these methods are practical under conditions which differ greatly from conditions associated with forming mineral fibers. The production of organic non-circular fibers can be facilitated by pressurization of the bushings, whereas pressurization of bushings containing molten glass present severe operating problems. The melting points of glass and organic compositions differ by 1500.degree. F. (815.degree. C.) or more. The mineral material of this invention will have a liquidus temperature greater than about 1200.degree. F. (649.degree. C.), whereas organic compositions soften and/or decompose at much lower temperatures.
The differences in physical characteristics can be clearly understood by comparing the ratio of viscosity-to-surface tension for glass with the same ratio for organic fiber forming material. The viscosity-to-surface tension ratio (poises/(dynes/cm)) of polymers lies within the range of from about 25 to about 5000. The ratio for glass is within the range of from about 0.1 to about 25, preferably within the range of from about 0.25 to about 15, and most preferbly within the range of from about 0.4 to about 10. The viscosity of molten glass at fiber forming temepratures is typically about 300 poises whereas the viscosity of the molten organic material is typically on the order of about 1000 to about 3000 poises. Also, the surface tension forces of glass (on the order of about 250 to about 300 dynes/cm) are an order of magnitude greater than those of the organic material (about 30 dynes/cm). The lower viscosity and higher surface tension of glass make it about 100 times more difficult to prevent the shaped glass fibers from re-forming into glass fibers having circular cross-sections.
In spite of past attempts to manufacture non-circular mineral fibers, there has never been a commercially successful method or apparatus for achieving the goal of making non-circular fibers from non-circular orifices.